As is explained in U.S. Pat. No. 3,758,669, the formation of nitrides from massive pieces of actinide metals involves problems associated with the highly exothermic nitriding reaction. Of particular interest is the formation of uranium nitride from massive uranium metal.
One prior-art technique of forming uranium nitride from massive uranium metal is to hydride and dehydride the massive uranium metal until the entire mass of metal is broken down into uranium metal powder. The uranium metal powder thus formed is subjected to a controlled nitrogen atmosphere in order to convert the metal to the nitride. Due to the highly exothermic nitriding reaction, the temperature and rate of reaction are exceedingly difficult to control. Even under the very best conditions, there are always areas within the uranium metal which react too vigorously with the excess heat melting adjacent areas of uranium metal. The melted areas show up as agglomerates which must be separated and reprocessed. The process is generally dangerous and requires 3 to 4 weeks to convert a 1- to 1-1/2-kilogram mass.
One method of overcoming the problems of the above prior art is disclosed in U.S. Pat. No. 3,758,669. That process comprises first forming uranium hydride powder from the massive uranium metal in a conventional manner and then incrementally dehydriding and nitriding the dehydrided portion during a cyclic heating process. This process overcomes the problems associated with the highly exothermic reaction and is complete within about four days or less. The uranium nitride produced by this process sinters to about 95 percent of theoretical density with isolated porosity. However, due to the high density and non-connected porosity of this material, fission product gases, basically helium, produced during irradiation cause swelling and dimensional changes within the sintered specimens.